Medical devices to be inserted into a living body, such as catheters and guide wires, should exhibit excellent lubricity to reduce damage to living body tissue such as blood vessel and to enhance operability for the operator. For this purpose, a method of coating a base material surface with a hydrophilic polymer having lubricity has been developed and put to practical use. In such medical devices, elution or peeling of the hydrophilic polymer from the base material surface imposes a problem, from the viewpoint of retention of safety and/or operability.
To avoid such a problem, U.S. Pat. No. 5,670,558 discloses a medical device in which a surface lubricating layer is formed on a base material surface by a method in which the base material surface is impregnated with a polymer solution containing a mixture of a hydrophilic polymer having a reactive functional group in its molecule and a polymer having a functional group capable of reacting with the reactive functional group, and thereafter the polymers are allowed to react with each other to form a cross-linked structure, thereby forming an insoluble polymer. According to the method described in U.S. Pat. No. 5,670,558, the surface lubricating layer can be fixed on the base material firmly to a certain extent. Especially, in the case where the base material itself is swelled with the hydrophilic polymer solution, the base material and the hydrophilic polymer constituting the surface lubricating layer form an interpenetrating network structure, whereby firm fixation can be achieved.
According to the technology described in U.S. Pat. No. 5,670,558, however, there is the following problem. When the base material is not easily swelled with the hydrophilic polymer solution, the hydrophilic polymer constituting the surface lubricating layer is fixed on the base material by only the insolubilization owing to cross-linking or the like. Therefore, the possibility of peeling of the surface lubricating layer in this case is higher as compared with the case of a base material capable of forming the interpenetrating network structure by being swelled with the hydrophilic polymer solution. Polyamide resin is a representative resin for wide use as base material for medical devices such as catheters, guide wires and indwelling needles. Polyamide resin is a crystalline polymer in which a hydrogen bond can be formed between amide linkages of different polymer chains, and strong intermolecular forces are acting in the crystalline region. Therefore, the polyamide resin is not easily swelled with a hydrophilic polymer solution, and it is difficult to form an interpenetrating network structure. Accordingly, the surface lubricating layer is formed only by the insolubilization of the hydrophilic polymer. Thus, the problem of easy peeling of the surface lubricating layer has existed. There is thus a need for a method by which a hydrophilic polymer can be more firmly fixed on a polyamide surface that is not easily swellable with a hydrophilic polymer solution.